Since an inorganic glass has excellent general properties such as excellent transparency and low optical anisotropy, the inorganic glass has been widely used in many fields as a transparent material. However, the inorganic glass has drawbacks such that it is heavy and easily broken, and has bad productivity when producing a product by molding and processing. As a result, a transparent organic polymer material (optical resin) has been used as a material in place of the inorganic glass. As the optical component obtained from such an optical resin, there are, for example, a spectacle lens for vision correction, and a plastic lens such as a camera lens of a digital camera and the like. The optical members have been put to practical use and have come into use. In particular, for the purpose of use in a spectacle lens for vision correction, the organic polymer material is lightweight and hardly broken, and can be dyed for granting great fashionability, as compared to the lens made of an inorganic glass. Making good use of such merits, the organic polymer material has been widely used.
In the past, a crosslinking type resin obtained by casting polymerization of diethylene glycol bisallylcarbonate as an optical resin used for a spectacle lens under heating (hereinafter commonly referred to as a DAC resin) has been put to practical use. The crosslinking type resin has merits such that transparency and heat resistance are excellent, and the chromatic aberration is low. Due to such merits, it has been used the most for a general-purpose plastic spectacle lens for vision correction. However, the central or peripheral thickness (edge thickness) of the plastic lens becomes large because of the low refractive index (nd=1.50). For that reason, there are problems such that wearing comfort and fashionability are worsened and the like. Therefore, a resin for a plastic lens with a high refractive index capable of solving these problems has been demanded and developed accordingly.
In such a trend, highly superior characteristics have been achieved such that polythiourethane comprising a sulfur atom obtained by casting polymerization of a diisocyanate compound with a polythiol compound is excellent in its transparency and impact resistance, while attaining a high refractive index (nd=1.6 to 1.7) and having relatively low chromatic aberration. Such polythiourethane has been used for the purpose of a high-quality plastic spectacle lens for vision correction in which the thickness is thin and its weight is light.
On the other hand, in a trend to pursue an optical resin having a much higher refractive index, there have been proposed several resins such as a transparent resin obtained by polymerization of a compound having an episulfide group in Patent Documents 1 and 2 or a compound having a thietane group in Patent Document 3, a resin obtained by polymerization of a metal-containing compound such as Se or the like in Patent Documents 4, and the like. However, the transparent resin obtained by polymerization of a compound having an episulfide group has a problem in mechanical properties, the compound having a thietane group has a problem in its polymerization ability, and the resin obtained by polymerization of a metal-containing compound such as Se or the like has a problem in safety. So, there have been demanded for further improvements in such resins. In recent years, there has been demanded an optical resin having required general properties (transparency, thermal properties, mechanical properties and the like) as a plastic lens, while attaining a much higher refractive index (nd), for example, exceeding 1.7. The development of such an optical resin has been made. Under these circumstances, a metal-containing thietane compound has been newly found and an optical resin having a high refractive index (nd) of exceeding 1.7 has been proposed in Patent Document 5.
Patent Document 1: Japanese Patent Laid-open No. 1997-110979
Patent Document 2: Japanese Patent Laid-open No. 1999-322930
Patent Document 3: Japanese Patent Laid-open No. 2003-327583
Patent Document 4: Japanese Patent Laid-open No. 1999-140046
Patent Document 5: International Publication Pamphlet No. 2005/095490